Ensure we evaluate VLA bounds if a variably-modified type is used as the
argument to __builtin_va_arg. Patch by Rahul Jain, some test massaging and
IR emission order changes by me.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206223 91177308-0d34-0410-b5e6-96231b3b80d8